The invention relates to a process for the partial combustion of a finely divided solid fuel, such as pulverized coal, in which the latter is introduced together with oxygen-containing gas via a burner into a reactor or gasifier from which a stream of high-temperature raw synthesis gas is discharged together with a minor amount of contaminating material, most of which is in the form of particles of fly ash.
Partial combustion is the reaction of all of the fuel particles with a substoichiometrical amount of oxygen, either introduced in pure form or admixed with other gases, such as a transport stream of nitrogen, whereby the fuel is partially oxidized to predominantly hydrogen and carbon monoxide. This partial combustion differs from complete combustion wherein the fuel would be completely oxidized to carbon dioxide and water.
During the process of partial combustion of pulverized coal in a gasifier, the mineral matter in the coal splits into two streams when the coal is gasified. Molten slag which is formed falls to the bottom of the gasifier where it is discharged. Lightweight particles of fly ash or fly slag which also are formed are carried out through the top of the gasifier by the stream of synthesis gas which is piped through a quench section and thence to a gas cooler, heat exchanger or waste heat boiler where steam may be generated.
The product gas and fly ash pass through equipment at high pressures, say 300 to 350 psig for example. The fly ash must then be separated from the product gas, collected, depressurized, purged of product and/or toxic gases, cooled, and converted to a form for easy disposal.
An essential component of this process is a means for repeatedly isolating the particulate solids container, i.e., a lock hopper, for filling and emptying. Thus the lock hopper is (a) raised to an elevated pressure, e.g., 350 psig, and filled; (b) depressurized to transfer pressure; (c) emptied of particulate contents, then (d) isolated (purged) for repressurization, refilling and a repeat of the process. The depressurization and purging is generally performed at or near ambient pressure, i.e., the tank is vented to atmosphere thereby wasting this energy and material.
To conserve some of the stored energy and improve efficiency, a dual lock hopper arrangement is sometimes used, as in Assignee's pilot plant, wherein one lock hopper (filled and at high pressure) is purged into the second (empty, low pressure) lock hopper thereby equalizing the pressures. The first lock hopper is then isolated, depressurized to transfer pressures and the particulate contents emptied to a receiver. After emptying, the first lock hopper is purged to atmosphere for a repeat of the refilling process. After being equalized, the second hopper is further pressurized, filled, purged into the first hopper (thereby equalizing pressures), isolated, depressurized and emptied to the receiver. This process repeats itself when the second lock hopper is empty and thus utilizes some of the stored energy which otherwise would be wasted (as in a single lock hopper). The dual lock hopper system, while reducing the wasted energy, still must purge to atmosphere the residual (pressure-equalized) pressure thereby wasting the stored energy during and after pressure equalization. This process is referred to as "cross-pressurization." The present invention improves upon the prior art by utilizing at least a portion of this otherwise wasted energy.